I suppose it depends on how the Demandingness Objection is spelled out. If the complaint against consequentialism is just that, if true, it would mean that most or all people aren’t always doing all that they’re morally obliged to do, then, sure, that’s not convincing at all.

But most of the carefully worked-out versions of this objection that I’ve encountered are complaining about something different: that maximizing consequentialism allows no conceptual room for supererogatory conduct. The problem isn’t that there’s anything unintuitive with the idea that fallible people often or always fall short. It’s that there are positive intuitive reasons to believe that there are at least some cases where doing something would be especially meritorious but failing to do it wouldn’t a case of falling short. The problem that many philosophers who stress “demandingness” have with maximizing consequentialism isn’t that the positive existential claim (that there are some ways in which we fall short when we omit to do something meritorious), but rather with the negative universal claim (that there are no ways in which we ever fail to fall short when we omit to do something meritorious).

Here’s a theological example: most Christians believe that God’s decision to incarnate Himself, to take on the sins of the world, and to suffer anddie on the Cross, was morally gratuitous: it was an act of undeserved grace, which God was not morally obliged to do, but did out of love for the world. But it’s impossible to make sense of a claim like that if you’re a maximizing consequentialist; either doing all that would lead to the greatest compossible aggregate of good effects within God’s range of choice, or it would lead to something less than the greatest compossible aggregate of good effects within God’s range of choice. Presumably, since God is all-seeing and all-powerful, he’d know ahead of time whether this is the case and would be capable of choosing the better course if there were a better course. But then it would follow that God’s actions were either morally obligatory for Him, or else morally wrong, depending on the breaks of whether or not the action led to the greatest compossible aggregate of good effects. There’s no room for the claim that God might willingly choose to do something above and beyond His moral obligations, because by definition there is nothing above and beyond the maximum. Which would be a problem for Christian soteriology.

I’m not a Christian myself, but I do certainly think that there are supererogatory actions which people can, and have, carried out, even in this vale of tears. Which seems like a good reason to reject maximizing consequentialism.